1. Field of the Invention
The present invention relates to a crankshaft for an internal combustion engine, the crankshaft having a balancer weight and also to a method of producing the same.
2. Description of the Prior Art
Generally a crankshaft of an internal combustion engine has, as its essential parts, journal portions (referred to simply as "journals" herein) rotatably supported by bearings, a crank pin portion (referred to simply as a "crank pin" herein) connected to one end of a connecting rod which, in turn, is connected at its other end to a piston slidably positioned in a cylinder, and crank web portions (referred to simply as "crank webs" herein) for supporting the crank pin. These essential parts of a crankshaft are required to provide sufficient mechanical strength to withstand various forces, such as the force produced as a result of the combustion, and the inertia force produced by reciprocating movements of the piston, rotational movements of the crankshaft itself and the like. It is an ordinary technique to attach a balancer weight to the crank web in order to attain a balance of the inertia forces acting on the crankshaft. The balancer weight has to be designed to have a suitable shape for achieving a satisfactory balancing function.
In producing a crankshaft by forging, it is rather easy to obtain the required mechanical strength at the journals and the crank pin, but the shape of the balancer weight is undesirably limited due to the conditions imposed by the die splitting process. In addition, as shown in FIG. 1, it is difficult to completely fill the innermost portions 3 of the die cavities in the dies 1 and 2 with the material of the crankshaft blank which plastically flows during forging. For this reason, forging is not suitable for the production of crankshafts, particularly of thin-walled crank webs having large diameter.
On the other hand, the production of crankshafts by casting cannot provide crankshafts with mechanical strength comparable to that of the forged product, although it offers a greater degree of freedom in shape and, hence, permits the formation of a balancer weight of any desired shape.
Accordingly, it is a primary object of the invention to provide a crankshaft for an internal combustion engine which is improved to permit the formation of the balancer weight in any desired shape while ensuring sufficient mechanical strength of the crankshaft without substantial difficulty, as well as a method of producing such an improved crankshaft.
To this end, according to the invention, a crankshaft having a balancer weight is produced by forming the main parts of the crankshaft including journals, crank pins and crank webs from a steel and then forming and integrating a balancer weight with each crank web around the core part of the latter by casting.
The above and other objects, features and advantages of the invention will become clear from the following description of a preferred embodiment taken in conjunction with the accompanying drawings.